The present invention relates to a roller fixing unit for a color electrophotographic apparatus.
In a treatment process used in electrophotographic printing, there are a lot of methods of fixing a toner image which has been transferred onto a print medium, such as paper or plastic film. Among the methods, a method utilizing thermal energy has long been known.
In the method utilizing thermal energy, a toner image formed on a print medium is melted by heating so as to adhere to the medium, and for this purpose the toner image is generally pressed by a roller heated up to the temperature at which the material composing the toner becomes adhesive. Especially, when the print medium is paper, the toner is tightly fixed to the printing paper since the toner is melted and part of the toner is absorbed into the fiber of the paper.
A roller fixing unit employing a heated roller is typical among those fixing units utilizing thermal energy, and this type of unit includes a fixing roller and a pressing roller contacting the fixing roller, the print medium having a toner image on it being passed between the rollers to fix the toner. In the roller fixing unit, generally, a heater provided inside a roller is switched on/off by detecting the surface temperature of the roller using temperature detecting element provided near the surface of the roller.
In recent years, however, color toners used in a color electrophotographic apparatus have low softening temperatures compared to the black toner commonly used in a conventional electrophotographic apparatus. Therefore, the roller fixing unit for a color electrophotographic apparatus employs a roller composed of a cylindrical metallic core 5 of aluminum coated with silicon rubber 4 as a fixing roller 1, as shown in the schematic view in FIG. 1, since silicon rubber has excellent removability of toner.
However, since silicon rubber has a low thermal conductivity, the surface temperature of the fixing roller, therefore, largely varies with the passing-through of the print medium, as shown in FIG. 3, and so it has been difficult to maintain the surface at an optimum temperature for fixing the toner by a common control method using a temperature sensor. For this reason, there has been a problem in that image characteristics, such as coloring, density and luster largely vary, and so an image having uniform qualities is difficult to obtain. Especially in a full color printing in which yellow, magenta, cyan and black toners are superposed and melted so as to be fixed in order to reproduce colors at will, it is always required to make the melting degree of each color toner constant. Since the image characteristics are strongly dominated by the fixing temperature of the roller, the roller temperature control is extremely important.
Further, in the conventional roller fixing unit in which the heater is controlled by detecting the surface temperature of the fixing roller, heat on the roller surface in a standby state is removed by passing of the first print medium, resulting in a phenomenon that the surface temperature rapidly drops for a while (hereinafter referred to as "undershoot").
The phenomenon is caused by temperature drop only in a thin surface layer of the silicon rubber coating on the surface of the fixing roller due to the low thermal conductivity of silicon rubber, that is, the phenomenon results from apparent lack of heat in the fixing roller. However, in actual practice the heat stored in the entire roller is not so much decreased. Since a heater controlled on the basis of a roller surface temperature which is apparently lowered is supplied with current to supply the roller with heat energy, the heat in the entire roller is overly supplied. In addition to this, since the heat is conducted to the roller surface with a time lag due to the silicon rubber, there appears a phenomenon that the surface temperature increases higher than a desired set or target temperature (hereinafter referred to as "overshoot").
The undershoot described above causes degradation in image quality due to occurrence of a fixing fault. The overshoot causes offset in the print medium. Especially, since color toners have low softening temperatures, as described above, an offset in the print medium is apt to appear.
In order to solve the problems in the roller fixing unit, various countermeasures have been tried. As a typical fixing unit, the following unit is proposed (Japanese Patent Application Laid-Open No.54-29650 (1979)).
The roller fixing unit referred to above has a fixing roller and a pressing roller having elastic coatings on their surfaces, rotating in contact with each other, at least one of the rollers containing a heater and a temperature sensor in the core portion of the roller, the temperature of the core member being controlled so as to be kept approximately constant throughout a standby period and paper passing period, the temporary temperature drop in the roller surface due to paper passing being compensated with second heating means provided separately from the roller.
The second heating means described above is what heats the roller surface using an external heater, or heats it by causing a heating roller to contact the fixing roller.
The prior fixing unit needs the second heating means as described above, and the setting temperature needs to be always set a little higher than a target temperature. Further, it is important always to keep the unit in an operating state. Therefore, problems arise with regard to the complexity and large scale of the unit, as well from an increase in power consumption.